paypal optoins

I am intersted in buying but after paying with paypal ,i would like to know how to go about unscribing or stopping payments being taking out?
I am looking to know how to do this be4 i buy.,or if there is a option of a 1 month,3 month,ect ?

By paying manually via Authorize.net you can send as many 20$ payments as you wish, then post back all the transaction ID';s and your account will be credited for the total time your payments grant you